Pressure transducers are used widely for pressure measurement. An example prior art device is described in U.S. Pat. No. 4,576,181 and illustrated in FIG. 1A. Such devices require connection to a flow channel or chamber to provide fluid communication with a sensor portion. For example, a flow channel 32 of a prior art device provides fluid communication between a diaphragm 45 and a vessel or conduit 30 containing a fluid whose pressure is to be measured, from some flow or containment system 47. An intermediate fluid in a space 35 on an opposite side of the diaphragm 45 communicates with a pressure transducer 40. The fluid whose pressure is to be measured exerts a pressure on the diaphragm 45 in turn exerting a pressure on the intermediate fluid in space 35. A pressure transducer 40 generates a signal corresponding to the pressure of the intermediate fluid in the space 35 by any of various mechanisms, typically involving a strain gage or load cell.
Another known device for measuring pressure is illustrated in FIG. 1B. In this device, a thin plate 30 has a strain gage 10 on a back surface 31 thereof. A pliant thin-walled vessel 20 rests against a front surface 32 of the thin plate 30. When fluid 25 inside the vessel 20 pressurizes the vessel, which is bounded by walls 15 and 22, thin plate 30 flexes, stretching a strain gauge 10 attached to it, thereby causing a signal from which pressure can be correlated by calibration.
The pressure sensor of FIG. 1B may be employed in medical systems and devices that transport biological fluids. In such systems, the use of certain plastics is very common, due to its durability, flexibility, low cost, and low chemical and biological reactivity. Such plastics, however, when strained, are susceptible to change in terms of their elastic response.
For example, if substantially deformed, thicker walled plastic vessels such as 20 in FIG. 1B will exhibit a condition known as “creep”, causing the displacement-versus-pressure response to change over time. Creep is caused by changes in the conformation of polymer molecules over time. Creep may lead to errors in measurement of pressure changes in a configuration such as that of FIG. 1B.
Referring to FIG. 1C, another type of prior art pressure sensor in which a pressure transducer 50 is in pressure communication with an interior 70 of a drip chamber 60. Blood flows through an inlet tube 65 and out an outlet tube 75 while a trapped volume of air 62 communications pressure to the pressure transducer 50 through a coupling tube 57. An isolator 55 protects the pressure transducer 50 by preventing any flow through it via a flexible membrane within it (not shown).